The vertebrate immune system requires multiple signals to achieve optimal immune activation (see, e.g., Janeway, Cold Spring Harbor Symp. Quant. Biol. 1989; 54:1-14; Paul William E., ed. Raven Press, N.Y., Fundamental Immunology, 4th edition (1998), particularly chapters 12 and 13, pages 411 to 478). Interactions between T lymphocytes (T cells) and antigen presenting cells (APC) are essential to the immune response. Levels of many cohesive molecules found on T cells and APC's increase during an immune response (Springer et al., A. Rev. Immunol. 1987; 5:223-252; Shaw and Shimuzu, Current Opinion in Immnunology, 1988 Eds. Kindt and Long, 1:92-97; and Hemler, Immunology Today 1988; 9:109-113). Increased levels of these molecules may help explain why activated APC's are more effective at stimulating antigen-specific T cell proliferation than are resting APC's (Kaiuchi et al., J. Immunol. 1983; 131:109-114; Kreiger et al., J. Immunol. 1985; 135:2937-2945; McKenzie, J. Immunol. 1988; 141:2907-2911; and Hawrylowicz and Unanue, J. Immunol. 1988; 141:4083-4088).
T cell immune response is a complex process that involves cell-cell interactions (Springer et al., A. Rev. Immunol. 1987; 5:223-252), particularly between T and accessory cells such as APC's, and production of soluble immune mediators (cytokines or lymphokines) (Dinarello, New Engl. J. Med 1987; 317:940-945; Sallusto, J. Exp. Med. 1997; 179:1109-1118). This response is regulated by several T-cell surface receptors, including the T-cell receptor complex (Weiss, Ann. Rev. Immunol. 1986; 4:593-619) and other “accessory” surface molecules (Allison, Curr. Opin. Immunol. 1994; 6:414-419; Springer, 1987, supra). Many of these accessory molecules are naturally occurring cell surface differentiation (CD) antigens defined by the reactivity of monoclonal antibodies on the surface of cells (McMichael, Ed., Leukocyte Typing III, Oxford Univ. Press, Oxford, N.Y., 1987).
CD28 antigen, a homodimeric glycoprotein of the immunoglobulin superfamily (Aruffo and Seed, Proc. Natl. Acad. Sci. 1987; 84:8573-8577), is an accessory molecule found on most mature human T cells (Damle et al., J. Immunol. 1983; 131:2296-2300). Current evidence suggests that this molecule functions in an alternative T cell activation pathway distinct from that initiated by the T-cell receptor complex (June et al., Mol. Cell. Biol. 1987; 7:4472-4481). Monoclonal antibodies (MAbs) reactive with CD28 antigen can augment T cell responses initiated by various polyclonal stimuli (reviewed by June et al., supra). These stimulatory effects may result from MAb-induced cytokine production (Thompson et al., Proc. Natl. Acad. Sci 1989; 86:1333-1337; and Lindsten et al., Science 1989; 244:339-343) as a consequence of increased mRNA stabilization (Lindsten et al., 1989, supra).
CTLA-4 is accepted as opposing CD28 activity and dampening T cell activation (Krummel, J. Exp. Med. 1995; 182:459-465; Krummel et al., Int'l Immunol. 1996; 8:519-523; Chambers et al., Immunity. 1997; 7:885-895). CTLA-4 deficient mice suffer from massive lymphoproliferation (Chambers et al., supra). It has been reported that CTLA-4 blockade augments T cell responses in vitro (Walunas et al., Immunity. 1994; 1:405-413) and in vivo (Kearney, J. Immunol. 1995; 155:1032-1036), exacerbates antitumor immunity (Leach, Science 1996; 271:1734-1736), and enhances an induced autoimmune disease (Luhder, J Exp. Med. 1998; 187:427-432). It has also been reported that CTLA-4 has an alternative or additional impact on the initial character of the T cell immune response (Chambers, Curr. Opin. Immunol. 1997; 9:396-404; Bluestone, J. Immunol. 1997; 158:1989-1993; Thompson, Immunity 1997; 7:445-450). This is consistent with the observation that some autoimmune patients have autoantibodies to CTLA-4. It is possible that CTLA-4 blocking autoantibodies play a pathogenic role in these patients (Matsui, J. Immunol. 1999; 162:4328-4335).
Non-human CTLA-4 antibodies have been used in the various studies discussed above. Furthermore, human antibodies against human CTLA-4 have been described as immunostimulation modulators in a number of disease conditions, such as treating or preventing viral and bacterial infection and for treating cancer (e.g., PCT Publication WO 01/14424 and PCT Publication WO 00/37504). U.S. Pat. No. 5,855,887 discloses a method of increasing the response of a mammalian T cell to antigenic stimulation by combining a T cell with a CTLA-4 blocking agent. U.S. Pat. No. 5,811,097 discloses a method of decreasing the growth of non-T cell tumors by administering a CTLA-4 blocking agent. U.S. patent application Ser. Nos. 09/644,668 and 09/948,939 disclose human CTLA-4 antibodies. Each of these patents and applications is hereby incorporated by reference.
The citation or discussion of any reference in this section or elsewhere in the specification is made only to clarify the description of the present invention and is not an admission that any such reference is “prior art” to any invention described herein.